This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. Ser. No. 100 20 228.4 filed in Germany on Apr. 25, 2000; the entire content of which is hereby incorporated by reference.
The present invention relates to the field of high-voltage insulation. It relates in particular to a high-voltage insulation system for electrical insulation of components whose operating temperature is below room temperature, and a method for producing such a system.
For use in the field of electrical power supply with system voltages of up to 550 kV, a high-voltage insulation system which is suitable for low temperatures is required for electrical parts or components which are intended to be used primarily at an operating temperature below room temperature. A combination of a coolant and solid material insulation is often used for this purpose. If the envisaged operating temperatures are sufficiently low, chemical aging processes as degradation mechanisms for the solid material insulation can virtually be ruled out. On the other hand, thermal stresses are caused in the insulation material as a result of the difference between the manufacturing temperature and the operating temperature, which may lead to damage such as cracks or de-lamination when cooled down and heated up frequently. If the electrical parts or components are in direct mechanical contact with the solid material insulation, the thermal co-efficient of expansion of the insulation must, furthermore, not differ excessively from that of the component, in order to avoid stresses in the latter.
Electrical parts having components based on high-temperature superconductors, for example, cables, transformers, current limiters and the like, are of particular interest. Liquid nitrogen (LN2) is preferably used for cooling high-temperature superconductors to operating temperatures below 80 K.
The solid material insulation which is used is also generally intended to have a certain mechanical robustness and to be capable of acting as a support or stabilizer for, for example, components composed of ceramic high-temperature superconductor material. Insulation composed of polymer films or sulfate paper is not suitable for use in these circumstances. Insulation components which can be stressed mechanically are normally produced from glass-fiber-reinforced fiber composite materials. The latter contain a polymer matrix composed of cured epoxy resin and glass fibers or carbon fibers as the reinforcing base material. Fiber composite materials containing glass fibers have a low partial discharge inception field of  greater than  greater than 1 kV/mm at 77 K, however, and even if special vacuum-pressure impregnation methods are used for casting the resin compound, the best that can be achieved is  greater than  greater than 4 kV/mm. Accordingly, in order to avoid excessive field strengths, the insulation must not be less than a certain minimum thickness, which is not consistent with efforts to achieve compact dimensions.
Pressboards, i.e. compressed boards produced from cellulose are frequently used for insulation of transformers and are in widespread use, for example, under the name xe2x80x9cTransformerboardxe2x80x9d. These are available in thicknesses from 0.5 mm to a few mm and, in laminated and bonded form, up to more than 100 mm. U.S. Pat. No. 3,710,293 discloses an insulation system comprising layers of pressboards and sulfate or kraft paper, which are cast using a thermoplastic resin. As an alternative to this, solid material insulation impregnated with oil and composed of cellulose paper is used to form barriers between adjacent winding layers in oil-cooled transformers. First, however, the former has to be dried by means of a complex heat-treatment and vacuum method. This is intended to prevent the cellulose material from releasing water to the oil and thus reducing its dielectric characteristics.
An exemplary embodiment of the present invention provides a high-voltage insulation system for use at temperatures below room temperature and with a high partial discharge inception field, and specifies a method for producing such a system.
The essence of the invention is to use as an electrically insulating coolant in conjunction with solid material insulation in the form of a composite material, which comprises cellulose fibers impregnated with polymer resin. The increased partial discharge inception field of the polymer composite allows the high-voltage insulation system to have more compact dimensions and thus also results in cost savings.
According to a first preferred embodiment, liquid nitrogen LN2 is used as a coolant. LN2 is suitable for cooling high-temperature superconductors to an operating temperature of 77 K or less. In the range between room temperature and the operating temperature, the mean thermal coefficient of expansion of the cellulose polymer matrix composite is comparable to that of the high-temperature superconductor. This results in the possibility of bringing the cellulose composite and the high-temperature superconductor into direct and permanent mechanical contact without any need to be concerned about damage induced by stresses during cooling or heating.
In order to allow the solid material insulator to provide mechanical support for the high-temperature superconductor ceramic, the cellulose material is advantageously used in the form of pressboards. In order to achieve greater thicknesses and further improve mechanical robustness, a number of thin boards, which can be formed individually, can be laminated. An intermediate layer composed of a suitable fabric absorbs excess polymer resin and prevents the formation of a pure resin layer between the pressboards.
An exemplary method according to the invention for producing a high-voltage insulation system which is suitable for low temperatures, is distinguished by the pressboards being formed in the dry state and then being impregnated, that is to say, soaked with a polymer resin. Since the process of forming the pressboards does not involve moistening them, there is also no need for the tedious drying process required for the subsequent impregnation. In consequence, there is no risk either of the formed pressboard becoming inadvertently distorted during the drying process.
According to further embodiment, a cylindrical coil former or coil support is formed from the pressboards, and a superconducting wire is then wound on it. The coil former and winding are then jointly encapsulated with polymer resin, which results in the windings being bonded and mechanically fixed to the coil former.